1. Field of Invention
The invention relates to an ultrasonic transducer with a housing, an ultrasonic window provided in a first region of the housing for transmitting ultrasonic waves between the interior of the ultrasonic transducer and the exterior of the ultrasonic transducer, and a transducer element located in the housing adjacent to the ultrasonic window, in which ultrasonic waves can be transmitted as housing waves between the first region of the housing by way of at least one intermediate second region of the housing and a third region of the housing which is opposite the first region of the housing.
2. Description of Related Art
Ultrasonic transducers of the aforementioned type have been known for years and are used, for example, to a large extent in acoustic flow rate measurement devices. The transducer element of the ultrasonic transducer converts electrical energy into a mechanical deflection, with suitable excitation also into an oscillation in the ultrasonic range. In this case, the ultrasonic transducer works as an ultrasonic transmitter and the ultrasonic waves are transmitted partially by way of the ultrasonic window into the medium which surrounds the ultrasonic transducer.
Conversely, it is also possible for the ultrasonic window to be deflected by external pressure fluctuations which occur in the medium and for the deflection to be converted by the transducer element into a corresponding signal; in this case, the ultrasonic transducer works as an ultrasonic receiver. In many applications, such as, for example, in fill level measurement, such an ultrasonic transducer is used both as an ultrasonic transmitter and an ultrasonic receiver. In the field of flow rate measurement, an ultrasonic transducer is often used either as an ultrasonic transmitter or as an ultrasonic receiver.
In both cases, therefore, both in the case in which the ultrasonic transducer works as a transmitter and also in the case in which the ultrasonic transducer works as a receiver, the ultrasonic waves, which have been transmitted by way of the ultrasonic window and which reach the transducer element or originate from the transducer element, are the actual useful signal of interest. The initially described ultrasonic waves which are relayed or diverted by way of the housing are parasitic housing waves. The energy transmitted with these waves is not available to the useful signal. Therefore, housing waves are generally undesirable.
There are various measures for reducing the housing waves. Some measures relate to the object of avoiding formation of these housing waves. They include, for example, certain configurations of the ultrasonic window with respect to especially good impedance matching for maximizing the transmitted energy portion or with respect to design of the ultrasonic window as a λ/4 layer for reducing reflections. Other measures relate to preventing already formed housing waves from being relayed, for example, by mismatched acoustic impedance transitions. However, housing waves constitute not only power lost for the useful signal, but moreover, they can have other adverse effects.
In acoustic flow rate measurement, for example, the effect is generally used that, in a medium transported in a measurement tube, the transport velocity of the medium is superimposed on the propagation velocity of the acoustic signal. The measured propagation velocity of the acoustic signal relative to the measurement tube is therefore larger than in a medium at rest. When the medium is being transported in the direction of the acoustic signal, the velocity of the acoustic signal relative to the measurement tube is less than in a medium at rest or when the medium is being transported opposite to the emission direction of the acoustic signal. The transit time of the acoustic signal between the acoustic transmitter and the acoustic receiver—both are ultrasonic transducers—as a result of the entrainment effect depends on the transport velocity of the medium relative to the measurement pipe, and thus, relative to the acoustic transmitter and the acoustic receiver.
In measurements which are based on emitted acoustic or ultrasonic signals, not only in the field of flow rate measurement, the problem is that the ultrasonic oscillations produced by the transducer element are transmitted not only by way of the ultrasonic window of the transducer housing into the surrounding medium of the ultrasonic transducer, but that the generated ultrasonic oscillations are transmitted in part also by way of the housing as housing waves to the entire measurement device. In addition to power loss, this is a problem since the ultrasonic waves transmitted by so-called crosstalk to the housing of the measurement device can also lead to considerable reception side interference. This is due to the fact that, on the receiving side it, cannot be easily distinguished whether the received ultrasonic signals have been received directly by way of the medium and the ultrasonic window—useful signal—or have traveled indirectly as housing waves by way of the measurement device and the entire housing of the ultrasonic transducer, especially therefore by way of the third region of the housing and the intermediate second region of the housing to the transducer element.